There are a variety of methods known for generally determining the health of gas turbine engines, particularly aircraft engines. General engine condition, as well as an indication of engine life expectancy and need for overhaul, is provided by trending systems which utilize engine parameters such as various temperatures, pressures and control parameters associated therewith to determine current engine operating condition and impute engine health. However, such systems do not recognize isolated events within the engine which can be indicative of severe engine distress or impending engine component failure from a particular causal event.
Certain engine conditions can be determined visually, such as through borescopes, without tearing down the engine. As examples, severe blade erosion (high temperature corrosion), loss of abradable seal segments, or excessive rubbing can frequently be detected by borescope inspection methods. Additonally, periodic teardown of an engine allows inspection of far more components, much more reliably. Because engine teardown is such a complex and expensive proposition, various schemes are employed to determine when engine teardown should be performed. Tearing engines down too frequently is, of course, an extreme waste of time and money. Failure to tear down an engine when it may have problems could result in engines malfunctioning while in use. Because of the complexity and expense involved, any improvement in diagnostic methodology is of great value.
In the past decade, monitoring of the electrical characteristics of gas flowing through a jet engine has been studied as a possible indication of engine deterioration. Apparatus disclosed in U.S. Pat. No. 3,775,763 utilizes an electrostatic probe positioned in the exhaust of the jet engine, such as through the tail pipe wall. Abnormal conditions were thought to be coupled with small particles striking the probe and causing spikes of ion current of a relatively large magnitude. Subsequently, as reported by Couch, R.P.: "Detecting Abnormal Turbine Engine Deterioration Using Electrostatic Methods", Journal of Aircraft, Vol. 15, October 1978, pp 692-695, it was theorized that the signals did not result from individual particles of metal hitting the probe, but rather that the signals were indicative of Trichel pulses (a form of repetitive corona discharge) created by high potential pockets of excess charge. A probe set including circular insulated segments within the gas turbine engine tail pipe and a triangle of wire extending through the tail pipe exhaust gas path were developed. With these probes, a normalized count of large signals (probe current, or voltage developed across an impedance, in excess of a threshold magnitude) over a period of time definitely correlated with impending engine component malfunctions or severe deterioration. As reported in the aforementioned article, however, the use of normalized counts of large magnitude signals from the ring and grid probe was thought to provide reliable prediction of only two out of three gas-path failures, at best, and distinction between possible causes thereof was highly experimental, as described below.
In a commonly owned, copending U.S. patent application entitled "Waveform Discriminated, Electrostatic Engine Diagnostics", Ser. No. 454,124, filed contemporaneously herewith by Zwicke et al, characteristics of the waveshape of signals developed on an electrostatic probe disposed for response to electrostatic charge in the gas stream of a gas turbine engine are utilized to discriminate waveshapes correlated to particular engine events or conditions, or classes thereof, from other waveshapes not having those characteristics or having other characteristics, which are thereby identified as different, whether correlated with other events and conditions or not.
In the waveshape-discriminated diagnostic system disclosed in the aforementioned application, it is shown that a number of electrostatic events in the gas stream of a gas turbine engine which are indicative of abnormal engine component wear cause electrostatic signals on more than one probe. It is also shown that there is a great deal of similarity between some signals on one probe that relate to particular engine events and other signals on the same probe that relate to different engine events. Naturally, the degree to which accurate waveform discrimination of electrostatic signals for correlation to particular engine events can be successful is dependent upon the degree to which similar pulses on the same probe can be discriminated.